A group of closely related peptide antibiotics naturally produced by Bacillus subtilis and Bacillus licheniformis are called Bacitracins. Bacitracins are generally active against many Gram positive and a few Gram negative bacteria species.
Several Bacitracins have been identified of which Bacitracin A is of primary importance and is highly active (Biochemistry, vol. 39 no 14, 2000, page 4037-45 by Epperson and Ming). Bacitracin A is a branched cyclic dodecapeptidolactam antibiotic that is synthesized via the thiotemplate mechanism of non-ribosomal peptide synthesis (J. Am. Chem. Soc. vol. 128, 2006, page 10513-10520 by Wagner et al and
Eur J Biochem, vol. 192 no. 1, 1990, page 1-15 by Kleinkauf and von Döhren).
The primary structure of Bacitracin A is NH2-L-Ile1-L-thiazoline2-L-Leu3-D-Glu4-L-Ile5-L-Lys6-D-Orn7-L-Ile8-D-Phe9-L-His10-D-Asp11-L-Asn12-COOH which is cyclized between the ε-amino group of L-Lys6 and the R-carboxyl group of L-Asn12 (J. Am. Chem. Soc. vol 128 page 10513-10520, 2006 by Wagner et al).
Ming et al (Journal of Inorganic Biochemistry, vol. 91, 2002) described Bacitracins as peptide compounds with the following formula:
wherein    X is Ile or Val and    Y is Ile or Val
Notably, R1 and R3 are the N-terminal moieties of active Bacitracins, while R4 and R5 are oxidized N-terminal moieties of less active Bacitracins. R2 is however an inactive stereoisomer.